TAF -
The international standard code format for terminal forecasts issued for airports, which took effect on I July 1996.

target -
Any of the many types of objects detected by radar. A radartarget must have an index of refraction sufficiently different from that of the atmosphere to return a target signal to the radar by reflection, refraction, or scattering. Also, it must be near enough and have a large enough radar cross section that the target signal will exceed the threshold of detectability of the radar receiver. The target is then said to produce a detectable echo.

TAS -
Abbreviation for true airspeed.

temperature -
The quantity measured by a thermometer. Bodies in thermal equilibrium with each other have the same temperature. In gaseous fluid dynamics, temperature represents molecular kinetic energy, which is then consistent with the equation of state and with definitions of pressure as the average force of molecular impacts and density as the total mass of molecules in a volume. For an ideal gas, temperature is the ratio of internal energy to the specificheat capacity at constant volume.

tendency -
The local rate of change of a vector or scalar quantity with time at a given point in space. Thus, in symbols, ¶[∂]p/¶[∂]t is the pressure tendency, ¶[∂]z[&zgr;]/¶[∂]t is the vorticity tendency, etc. Because of the difficulty of measuring instantaneous variations in the atmosphere, variations are usually obtained from the differences in magnitudes over a finite period of time and the definition of tendency is frequently broadened to include the local time variations so obtained. An example is the familiar three-hourly pressuretendency given in surface weather observations; in fact, the term "tendency" alone often means the pressure tendency.

thermal -
1. Pertaining to temperature or heat. 2. A discrete buoyant element in which the buoyancy is confined to a limited volume of fluid. See plume. 3. A relatively small-scale, rising current of air produced when the atmosphere is heated enough locally by the earth's surface to produce absoluteinstability in its lowest layers. The use of this term is usually reserved to denote those currents either too small and/or too dry to produce convective clouds; thus, thermals are a common source of low-level clear-air turbulence. It is generally believed that the term originated in glider flying, and it is still very commonly used in this reference.

thermal gradient -
1. (Or geothermal gradient.) According to Smithsonian Physical Tables, the rate of variation of temperature in soil and rock from the surface of the earth down to depths of the order of kilometers. It varies greatly from place to place, depending on the geological history of the region, the radioactivity of the underlying rocks, and the conductivity of the upper rocks. An average is about +10°C per km. 2. Same as temperature gradient. See also lapse rate.

thickness -
In synoptic meteorology, the vertical depth, measured in geometric or geopotential units, of a layer in the atmosphere bounded by surfaces of two different values of the same physical quantity, usually constant-pressure surfaces. See thickness chart.

thin -
As used in aviation weather observations, descriptive of a sky cover that is predominantly transparent. According to the summation principle, at any level, if the ratio of the transparent sky cover to the total sky cover (opaque plus transparent) is one-half or more, then the cloud layer at that level must be classified as "thin." It is denoted by the symbol "-" preceding the appropriate sky cover symbol.

THIR -
Abbreviation for Temperature-Humidity Infrared Radiometer.

thunder -
The sound emitted by rapidly expanding gases along the channel of a lightning discharge. Some three-fourths of the electrical energy of a lightning discharge is expended, via ion-molecule collisions, in heating the atmospheric gases in and immediately around the luminous channel. In a few tens of microseconds, the channel rises to a local temperature of the order of 10 000°C, with the result that a violent quasi-cylindrical pressurewave is sent out, followed by a succession of rarefactions and compressions induced by the inherent elasticity of the air. These compressions are heard as thunder. Most of the sonic energy results from the return streamers of each individual lightning stroke, but an initial tearing sound is produced by the stepped leader; and the sharp click or crack heard at very close range, just prior to the main crash of thunder, is caused by the ground streamer ascending to meet the stepped leader of the first stroke. Thunder is seldom heard at points farther than 15 miles from the lightning discharge, with 25 miles an approximate upper limit, and 10 miles a fairly typical value of the range of audibility. At such distances, thunder has the characteristic rumbling sound of very low pitch. The pitch is low when heard at large distances only because of the strong attenuation of the high-frequency components of the original sound. The rumbling results chiefly from the varying arrival times of the sound waves emitted by the portions of the sinuous lightningchannel that are located at varying distances from the observer, and secondarily from echoing and from the multiplicity of the strokes of a composite flash. See electrometeor.

thunderstorm -
(Sometimes called electrical storm.) In general, a local storm, invariably produced by a cumulonimbus cloud and always accompanied by lightning and thunder, usually with strong gusts of wind, heavy rain, and sometimes with hail. It is usually of short duration, seldom over two hours for any one storm. A thunderstorm is a consequence of atmospheric instability and constitutes, loosely, an overturning of air layers in order to achieve a more stable density stratification. A strong convective updraft is a distinguishing feature of this storm in its early phases. A strong downdraft in a column of precipitation marks its dissipating stages. Thunderstorms often build to altitudes of 40 000-50 000 ft in midlatitudes and to even greater heights in the Tropics; only the great stability of the lower stratosphere limits their upward growth. A unique quality of thunderstorms is their striking electrical activity. The study of thunderstorm electricity includes not only lightning phenomena per se but all of the complexities of thunderstorm charge separation and all charge distribution within the realm of thunderstorm influence. In U.S. weather observing procedure, a thunderstorm is reported whenever thunder is heard at the station; it is reported on regularly scheduled observations if thunder is heard within 15 minutes preceding the observation. Thunderstorms are reported as light, medium, or heavy according to 1) the nature of the lightning and thunder; 2) the type and intensity of the precipitation, if any; 3) the speed and gustiness of the wind; 4) the appearance of the clouds; and 5) the effect upon surface temperature. From the viewpoint of the synoptic meteorologist, thunderstorms may be classified by the nature of the overall weather situation, such as airmass thunderstorm, frontal thunderstorm, and squall-line thunderstorm.

TID -
Abbreviation for traveling ionospheric disturbances.

tide -
1. The periodic rising and falling of the earth's oceans and atmosphere. It results from the tide-producing forces of the moon and sun acting upon the rotating earth. This disturbance actually propagates as a wave through the atmosphere and along the surface of the waters of the earth. Atmospheric tides are always so designated, whereas the term "tide" alone commonly implies the oceanic variety. Sometimes, the consequent horizontal movement of water along the coastlines is also called "tide," but it is preferable to designate the latter as tidal current, reserving the name tide for the vertical wavelike movement. See equatorial tide, neap tide, spring tide, tropic tide. 2. See rip current, red tide, storm tide.

time -
Duration as measured by some clock. Atomic clocks give the most accurate measure of time. Less regular timekeepers are those based on the rotation of the earth and other bodies of the solar system.

tongue -
In oceanography, a three-dimensional, tonguelike intrusion of finite extent in the along-front direction. See interleaving.

topography -
1. Generally, the disposition of the major natural and man-made physical features of the earth's surface, such as would be entered on a map. This may include forests, rivers, highways, bridges, etc., as well as contour lines of elevation, although the term is often used to denote elevation characteristics (particularly orographic features) alone. 2. The study or process of topographic mapping.

tornado -
1. A violently rotating column of air, in contact with the ground, either pendant from a cumuliform cloud or underneath a cumuliform cloud, and often (but not always) visible as a funnel cloud. When tornadoes do occur without any visible funnel cloud, debris at the surface is usually the indication of the existence of an intense circulation in contact with the ground. On a local scale, the tornado is the most intense of all atmospheric circulations. Its vortex, typically a few hundred meters in diameter, usually rotates cyclonically (on rare occasions anticyclonically rotating tornadoes have been observed) with wind speeds as low as 18 m s-1 (40 mph) to wind speeds as high as 135 m s-1 (300 mph). Wind speeds are sometimes estimated on the basis of wind damage using the Fujita scale. Some tornadoes may also contain secondary vortices (suction vortices). Tornadoes occur on all continents but are most common in the United States, where the average number of reported tornadoes is roughly 1000 per year, with the majority of them on the central plains and in the southeastern states (see Tornado Alley). They can occur throughout the year at any time of day. In the central plains of the United States they are most frequent in spring during the late afternoon. See also supercell tornado, nonsupercell tornado, gustnado, landspout, waterspout. 2. A violent thundersquall in West Africa and adjacent Atlantic waters.

towering -
A mirage in which the angular height of the image is greater than that of the object. As the width is unaffected (the angular width of image width remains that of the object because the refractive indexgradient is vertical), the aspect ratio is altered and distant images appear vertically enlarged. Towering often accompanies sinking¾[—]distant features appear depressed and enlarged¾[—]but it can also accompany looming. Compare stooping.

trace -
1. In general, an unmeasurable (less than 0.01 in.) quantity of precipitation. 2. An insignificantly small quantity. 3. The record made by any self-registering instrument. Thus, one may speak of the barograph trace, the hygrothermograph trace, etc.

trade -
1. See trade winds. 2. Of or pertaining to the trade winds or the region in which the trade winds are found.

trajectory -
(Or path.) A curve in space tracing the points successively occupied by a particle in motion. At any given instant the velocity vector of the particle is tangent to the trajectory. In steady-state flow, the trajectories and streamlines of the fluid parcels are identical. Otherwise, the curvature of the trajectory KT is related to the curvature of the streamline KS by KT = KS - , where V is the parcel speed and ¶[∂]y[&psgr;]/¶[∂]t is the local change of the wind direction. The curvatures and wind change are positive for the cyclonic sense of flow.

transport -
The movement of a substance or characteristic. Characteristics that can be transported in the atmosphere are heat (temperature), moisture, momentum, chemicals, turbulence, etc. The transport is sometimes interpreted as a fluxdensity (characteristic per unit area per time), or as a flow rate (characteristic per time). See transport processes.

tropical air -
A type of air mass with characteristics developed over low latitudes. Maritime tropical air (mT), the principal type, is produced over the tropical and subtropical seas. It is very warm and humid and is frequently carried poleward on the western flanks of the subtropical highs. Continental tropical air (cT) is produced over subtropical arid regions and is hot and very dry.See airmass classification, trade air; compare polar air.

tropical disturbance -
A migratory, organized region of convective showers and thunderstorms in the Tropics that maintains its identity for at least 24 hours but has no closed wind circulation. The system may or may not be associated with a detectable perturbation of the low-level wind or pressure field.

tropical storm -
See tropical cyclone.

tropical upper-tropospheric trough -
A semipermanent trough extending east-northeast to west-southwest from about 35°N in the eastern Pacific to about 15°-20°N in the central west Pacific. A similar structure exists over the Atlantic Ocean, where the meantrough extends from Cuba toward Spain.

Tropics -
1. Any portion of the earth characterized by a tropical climate. 2. Same as Torrid Zone. See Tropic of Cancer, Tropic of Capricorn.

tropopause -
The boundary between the troposphere and stratosphere, usually characterized by an abrupt change of lapse rate. The change is in the direction of increased atmospheric stability from regions below to regions above the tropopause. Its height varies from 15 to 20 km (9 to 12 miles) in the Tropics to about 10 km (6 miles) in polar regions. In polar regions in winter it is often difficult or impossible to determine just where the tropopause lies, since under some conditions there is no abrupt change in lapse rate at any height. It has become apparent that the tropopause consists of several discrete, overlapping "leaves," a multiple tropopause, rather than a single continuous surface. In general, the leaves descend, step-wise, from the equator to the poles.

troposphere -
That portion of the atmosphere from the earth's surface to the tropopause; that is, the lowest 10-20 km (6-12 mi) of the atmosphere; the portion of the atmosphere where most weather occurs. The troposphere is characterized by decreasing temperature with height, appreciable vertical wind motion, appreciable water vapor, and weather. Dynamically, the troposphere can be divided into the following layers: surface boundary layer, Ekman layer, and free atmosphere. See atmospheric shell.

trough -
In meteorology, an elongated area of relatively low atmospheric pressure; the opposite of a ridge. The axis of a trough is the trough line. This term is commonly used to distinguish the previous condition from the closed circulation of a low (or cyclone), but a large-scale trough may include one or more lows, an upper-airtrough may be associated with a lower-level low, and a low may have one or more distinct troughs radiating from it. See front, dynamic trough, easterly wave, equatorial wave.

turbulence -
1. Irregular fluctuations occurring in fluid motions. It is characteristic of turbulence that the fluctuations occur in all three velocity components and are unpredictable in detail; however, statistically distinct properties of the turbulence can be identified and profitably analyzed. Turbulence exhibits a broad range of spatial and temporal scales resulting in efficient mixing of fluid properties. Analysis reveals that the kinetic energy of turbulence flows from the larger spatial scales to smaller and smaller scales and ultimately is transformed by molecular (viscous) dissipation to thermal energy. Therefore, to maintain turbulence, kinetic energy must be supplied at the larger scales. See also ocean mixing. 2. Random and continuously changing air motions that are superposed on the mean motion of the air. See aircraft turbulence.

type -
1. A specific classification of aircraft having the same basic design, including all modifications that result in a change in handling or flight characteristics. 2. See weather type.